The present invention relates to an X-ray CT image reconstruction method and an X-ray CT system in which fan-beam data concerning an X-ray beam that fans out is converted into parallel-beam data concerning parallel X-rays, which are supposed to be projected at an equal angle of projection, in order to reconstruct an image.
Recently, X-ray CT systems use three-dimensional tomographic image data of a subject to produce information on a projection image formed by projecting the three-dimensional tomographic image data in one direction. Methods for producing the information on a projection image include a maximum intensity projection (MIP) method of visualizing maximum pixel values detected in a direction of projection (refer to, for example, Non-patent Document 1).
When the MIP method is adopted, striped artifacts appear on a projection image. In efforts to minimize the striped artifacts, fan-beam data concerning an X-ray beam that fans out is converted into parallel-beam data concerning an X-ray beam, which is supposed to include parallel X-rays, in units of a projection line. The parallel-beam data is used to reconstruct an image (refer to, for example, Patent Document 1).
[Patent Document 1] Japanese Unexamined Patent Publication No. Sho 59(1984)-0168840 (pp. 3 and 4, FIGS. 2 and 3)
[Non-patent Document 1] “Radiological Modality Engineering” (Medical, Dental & Pharmacological Publishing, Apr. 20, 2003, pp. 174-175)
[Non-patent Document 2] “Image Processing Algorithm” (by Tsuneo Saitoh, Modern Science Publishing, Mar. 10, 1993, pp. 107-108)
However, according to the foregoing background art, the spatial resolution of a tomographic image gets poorer at a point in the image farther away from a point therein associated with a scan center position. In other words, the tomographic image becomes streamy towards the perimeter of a circle, of which center point is associated with the scan center position, proportionally to a distance from the point associated with the scan center position.
In particular, a tomographic image expressing the lung field radiographed through high-resolution CT examination contains an outstandingly streamy image in which blood capillaries existing in the lung field that are shown to be streamy towards the perimeter of a circle whose center is associated with a scan center position. Thus, the image quality is markedly poor.
Consequently, what counts with an X-ray CT image reconstruction method and an X-ray CT system is whether or not to be able to reduce a decrease in a resolution, which gets poorer proportionally to an increase in a distance of a point in a tomographic image from a point therein associated with a scan center position, occurring in case projection lines are converted from one form to another.